Field of the Invention
The present invention relates to a slip and a process for producing an aluminum diffusion layer.
Discussion of Background Information
In metallic components which are exposed to high temperatures, for example components of flow machines in the form of stationary gas turbines or aircraft engines, the metallic surfaces have to be protected from oxidative and/or corrosive attack. For this purpose, many different layer systems are known in the prior art.
Layer systems used also include, inter alia, diffusion layers in which chemical elements diffuse into the metallic surface to be protected and/or deposit on the surface in order to achieve an accumulation of the corresponding elements at the surface to be protected. The element which has diffused in then provides, together with the alloy constituents of the base material, appropriate properties in order to be able to operate the component at the desired high temperatures.
Thus, for example, the provision of aluminum diffusion layers on high-temperature materials, e.g. iron-, cobalt- or nickel-based alloys, is known, with the accumulation of aluminum in the surface region of the corresponding alloys leading to a slowly growing aluminum oxide layer being formed when high-temperature oxidative attack occurs and protecting the material against further damaging oxidative attack.
Such an aluminum diffusion layer can be produced, inter alia, by application of a slip comprising aluminum-containing powder particles which provide the aluminum for the diffusion process, where, after drying and/or hardening of the slip on the surface to be treated, the aluminum diffuses into the material from the dry slip layer during subsequent diffusion heat treatment. The application of such a slip by painting, dipping or spraying is very simple, so that a process of this type is of industrial interest for producing an aluminum diffusion layer. In particular, components can also be repaired in a simple way by the simple application.
Known slips for carrying out an aluminum diffusion process comprise, in addition to aluminum-containing powder particles, a binder which essentially provides the liquid phase for forming the slip. However, such a slip also has to be of such a nature that the aluminum-containing powder is preferably not oxidized by the binder, so that the subsequent diffusion process is not made difficult by the presence of aluminum oxide. Accordingly, provision of additional oxides such as silicon dioxide or else chromates, dichromates or phosphates in the aqueous and acidic binders is known. Chromates in particular have been used in the past in order to increase the corrosion resistance of the correspondingly treated metal component and also to inhibit oxidation of the metallic aluminum in the slip. However, chromium(VI) compounds are extremely toxic and hazardous to health, so that attempts are increasingly being made to replace these components in the slip compositions. Examples are described in EP 2 060 653 A2, U.S. Pat. No. 7,896,962 B2, WO 2010/134918 A1, U.S. Pat. No. 7,270,852 B2, U.S. Pat. No. 6,036,995 or WO 93/023247 A1.
In particular, attempts are made to produce stable slips for the production of aluminum diffusion layers which make do without toxic Cr(VI) compounds which are hazardous to health by the addition of silicon dioxide and the use of silicon-aluminum alloys as particle constituents in a slip and also the use of glycols as organic binders. Here, in particular, a passivating function for the metallic aluminum constituents is ascribed to the silicon as alloy constituent of the aluminum-containing powder and as additive in the form of silicon dioxide.
Although good results have already been achieved in this way, the problem that such slips do not have the required stability which enables the slip to be stored and processed over a prolonged period of time without decreases in the effectiveness of the slip during a subsequent diffusion heat treatment being observed remains.